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Considering Probes for Roughness Measurement

Skidded probes are available in two types: longitudinal and lateral. Here’s how to decide which is best for your surface finish measurement.

George Schuetz
George Schuetz
Director of Precision Gages, Mahr Inc.

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There are two basic principles of operation for probes used to measure surface finish: skidded and non-skidded. With skidded probes, the sensitive, diamond-tipped stylus is supported by a metal skid that rests on the workpiece. Thus, the workpiece itself is used as the reference surface. Non-skidded (or skidless) probes use an internal precision rail to provide the reference for the probe. This enables skidless probes to be used with high-end surface measuring systems for measurements of waviness and form parameters, in addition to roughness.
 
Most simple roughness measurements are made using a skidded probe. Many different types of skidded probes are available, and the characteristics of the part will most often determine which probe may be the best for the application. For example, some probes are designed to measure surface finish at the bottom of a groove or along a razor’s edge. The length of the surface to be measured can also dictate the style of probe.
 
Skidded probes are of two types: longitudinal skid-type probes and lateral skid-type probes.
 
• Longitudinal skid-type probes’ designs use a skid that runs in line with (coincident to) the length of the measurement probe. The skid surface is directly in front or behind the measurement stylus and contacts the surface to be measured. Depending on the manufacturer, the length of the skid surface before or behind the probe can vary, but it is typically around 0.120 inch. This means that the surface to be measured has to have a minimum length in order to support the skid. If the skid is not supported during measurement, the results will be erroneous.
 
• Lateral (transverse) skid-type probes are designed so that the skid runs across (or transverse) the length of the measurement probe. The skid surface is to the left or right of the probe. These types of probes are often a good alternative to longitudinal-type probes, because the short skid length can be used where the measurement land areas are short or the area is close to a shoulder.
 
However, due to the short skid length, the radius of the skid surface is also small. Again each manufacturer has its own probe design, but this type of probe typically has a skid radius of 0.012 inch, which can cause problems in certain applications.
 
Normally, the choice between these two probes depends on the length of the surface being measured. However, when measuring a surface using a short-skid radius in the transverse direction, it is also important to consider “Peak Spacing” (Sm) along with the roughness (Ra) value. It is not uncommon for a very rough surface to have larger peak spacing.
Checking the peak spacing helps ensure that the short-radius skid does not “fall in between” the actual peaks. Figure 1 demonstrates an exaggerated view of a measurement probe with a small skid radius measuring a surface with larger peak spacing. If the stylus radius falls between the peaks, mechanical filtering of the roughness values will yield measurements reading less than the actual values.
 

For measurement accuracy, the ideal situation is to have the skid on the measurement probe sit atop the peaks on the surface of the part, as shown in Figure 2. With the skid sitting atop the peaks, the stylus will travel the full dimension, from the top of the peak to the lowest valley. This ensures that no mechanical filtering of the measurement results occurs.  

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